The present invention relates generally to variable optical density lenses and in particular to lenses in which the optical density is controlled as a settable function of ambient light intensity.
Optical devices using liquid crystal lenses are known from, for example, U.S. Pat. No. 4,021,935, U.S. Pat. No. RE-29 684, U.S. Pat. Nos. 4,190,330 and 4,106,217. A type of variable optical density spectacle lens which responds to changes in ambient light is described in U.S. Pat. No. 4,279,474. The arrangement described therein uses a liquid crystal means sandwiched between a pair of matching curved glass lenses. This system gives a response time which is much faster than that of photochromic lenses. The rapid response of the liquid crystal lens overcomes the drawbacks of prior photochromic lens. The variation in optical density is achieved by driving the spectacle lens with an electrical signal of suitable waveform.
However, the response time of the liquid crystal lenses as described in U.S. Pat. No. 4,279,474 and other prior art is unlikely to represent the optimum for all users under all conditions. The actual response time of the lens will be determined by whichever of the response time of the liquid crystal or the time for the driving waveform to change is longer.
It is particularly important to achieve a satisfactory response time even in situations where, for example, the light falling on the photodetector is momentarily reduced and, with a short response time, the lenses would otherwise become maximally transmissive (optical density at a minimum), even though the general ambient level remains high. Practical cases of this are when driving on a sunny day through shadows cast by railings or a row of trees. At the same time, it is important still to provide for a fast response to changes in the ambient light level such as occur when a vehicle enters a tunnel or when the wearer of spectacles looks suddenly in the sun""s direction. Both requirements should be met for a commercially attractive product.
According to a first aspect of the present invention, there is provided an electrical circuit for controlling the optical density of a liquid crystal lens in dependence upon ambient light levels, in order to adjust the amount of light transmitted through the lens, the circuit including means for adjusting the response time of the circuit to changes in ambient light.
This provides the user with the ability to accommodate different lighting or use conditions and individual preferences and particularly allows the user to adjust the response time to cope with non-representative lighting situations which are suddenly and temporarily different from the general ambient conditions.
Further related considerations are the threshold light level at which the optical density starts to change and the rate of increase in optical density with excess light above this threshold. It is likely that the preferred threshold for response and the response characteristic above the threshold will vary both with latitude and season for example. In addition, for therapeutic applications, particularly photophobic conditions such as occur after cataract removal operations for example, both these parameters will need to be precisely controlled. Practical photodetectors are unlikely to have the required response characteristic.
Furthermore, it is desirable to cater for a wide range of individual preferences regarding the degree of darkening without having to provide a wide range of products to suit those individual preferences.
According to a second aspect of the invention therefore, there is provided an electrical circuit for controlling the optical density of a liquid crystal lens in dependence upon ambient light levels, in order to adjust the amount of light transmitted through the lens, the circuit including a photodetector, and means for adjusting the response of the circuit to excess light, above the threshold at which the optical density of the lens begins to change, in accordance with a desired response characteristic.
The invention also preferably includes means for adjusting the threshold of ambient light at which the circuit starts to adjust the optical density of the lens.
This is particularly useful in allowing for the variation of eye contraction and dilation which many people experience and which can be particularly acute for people with certain eye disorders.
The invention is thus able to provide a means for varying the rate at which the optical density changes in response to changes in ambient light, and, in addition, comprises an arrangement for setting the threshold at which darkening starts and a means for obtaining a desired darkening characteristic above the threshold.
The ambient light can be sensed by a photodetector, which may be a photodiode, photosensitive resistor or other photosensitive device, and a voltage is developed across a load resistor, or by using another circuit, which is a function of light level. In an analogue system the signal may be fed to an amplifying stage, where the gain is a function of signal level and a steady offset voltage may be added to set the threshold. The signal passes to a filter in which the time constant is set by a user adjustable control employing a potentiometer. The signal from the amplifying stage is fed to a voltage controlled oscillator. The duty cycle of the output from this oscillator depends on the control signal and the output is used to drive the lenses. The increase of optical density of the lenses depends on the duty cycle of the oscillator output, so that a desired response characteristic to light can be achieved by tailoring the gain characteristic of the amplifying stage. The oscillator pulse repetition rate is set so that the residual flicker of light transmitted through the lens is reduced to an acceptable level.
However, a digital circuit is preferred, in which case, the developed voltage can be fed to an analogue-to-digital converter, the output of which is passed to a signal conditioner and thence to a digital filter and a digitally controlled waveform generator which controls the liquid crystals of the lens.